It didn’t swallow anybody, but it's pretty weird and we don't know what caused it.

With no warning, a hellish rumble announces a crack in the ground, opening to a yawning chasm as the walls spread, crumble, and disappear into the abyss—fortunately, this particular seismic disaster occurs only in cartoons. (And ridiculous movies.) But tone down the special effects a bit and then try to put yourselves in the shoes of some residents of Michigan’s Upper Peninsula in October 2010.

Early in the morning, folks just north of Menominee, Michigan, heard a loud noise and felt a shake. In that part of the country, a grain elevator explosion is more likely than an earthquake. But when someone went out to finish cutting up a large tree that had come down in a storm two weeks previous, they found a huge crack had opened up in the Earth. It wasn’t going to swallow anybody whole, but you could probably have lost a cell phone in there.

The “Menominee Crack” was a little longer than a football field, over half a meter wide in places, and approached 1.7m deep. It ran through a forested area that had previously been flat. The crack actually sat atop what was now a six-foot-high ridge, with trees on either side now tipping slightly away from vertical. If you look carefully, you can actually see it in satellite imagery.

Geologists obviously came out pretty quickly to take a look—and to try to figure out what the heck happened. There were some good hypotheses, but plenty of head-scratching. In 2013, a group of Michigan Tech researchers led by Joshua Richardson went out to probe it with some seismic waves of their own. For small-scale investigations like this, the source of those seismic waves is a piece of steel smacked with a sledgehammer. Sensors placed around the crack detected seismic waves passing through, and bouncing off of, rock and sediment below the surface.

Limestone bedrock sits beneath about six feet of glacial sediment there. And seismic waves traveling through the bedrock perpendicular to the crack were a little slower than those traveling parallel to it, which is what you would expect if there were cracks in the rock, as well.

The cracks runs northwest-to-southeast. In the region, the dominant stress felt by the bedrock is a squeezing perpendicular to that. Connect the dots, and this looks like a classic example of what is called a “pop-up.” If you place a phone book on a table and press it down while someone else squeezes it from the sides… nothing will happen. But as soon as you remove the downward pressure, it will buckle, bulging upwards. Bedrock pop-ups are similar, except that the top layer of the rock cracks in half to form a shallow tent-like shape.

However, that requires the removal of downward pressure. Rock quarries can see pop-ups as rock is removed, but this wasn’t a rock quarry. The melting away of glacial ice has also been known to cause pop-ups, but not 11,000 years after the ice melted away. So why would a pop-up have occurred there in 2010?

The researchers don’t know. Is it possible that the retreat of the last ice sheet was nearly enough of a change to cause a pop-up here, but it had to wait for some other factor to put it over the top? Did groundwater dissolve a little of the limestone, weakening it? The researchers did note two things that were too coincidental to ignore. First, there’s the tree that had recently fallen in a storm. It was a big one, and it stood close to one end of the crack. Second, satellite photos from 1998 show a rectangular shape in the ground along the side of the road and ending near the crack. The only problem is that no one who lives there remembers what it was. Maybe it was a trench road crews borrowed dirt from at one point, or maybe it was just a small pasture or garden plot.

Without an obvious trigger for the bedrock pop-up, the researchers say it appears to be a first—they could find no other recorded examples of observed pop-up events outside of excavations like quarries. It’s a real Menominee anomaly.